1. Field of the Invention
The present invention relates to technology regarding the compatibility between CDs, DVDs, Blu-ray Discs, HD DVDs and next-generation high-density optical disks such as, near-field optical recording.
2. Description of the Related Art
With the practical application of blue-violet semiconductor lasers, Blu-ray Discs (abbr. “BD” hereinafter), which are high-density, large-capacity optical information recording media (called “optical disks” hereinafter) of the same size as CDs (Compact Discs) and DVDs (Digital Versatile Discs), have in recent years come into actual use. The BD is an optical disk that records or reproduces information using a blue-violet laser beam source. A HD DVD, which also uses a blue-violet laser beam source, has also recently come into use.
A CD is an optical disk having a transparent substrate 1.2 mm in thickness, with a single-layer storage capacity of approximately 650 MByte. The laser beam used for recording or reproduction of a CD has a wavelength of approximately 780 nm, and the NA (numerical aperture) of the objective lens is 0.45.
In order to record or reproduce information onto or from an optical disk of higher density, it is necessary to reduce the diameter of the beam spot condensed by the objective lens. Here, in order to reduce the diameter of the beam spot, it is necessary to shorten the wavelength λ and increase the numerical aperture NA of the objective lens. However, increasing the numerical aperture NA of the objective lens leads to an increase in coma aberration arising due to tilting of the transparent substrate. This coma aberration increases proportionally to a cube of the numerical aperture. The coma aberration is also proportional to the thickness of the transparent substrate. Therefore, the thickness of the transparent substrate may be reduced in order to suppress coma aberration.
A DVD is an optical disk having a transparent substrate 0.6 mm in thickness, with a single-layer storage capacity of approximately 4.7 Gbyte. The laser beam used for recording or reproduction of a DVD has a wavelength of approximately 650 nm, and the numerical aperture NA of the objective lens is 0.60. In a DVD, two 0.6 mm-thick substrates are layered together, and thus the total thickness of the optical disk is 1.2 mm, or the same as a CD.
A BD is an optical disk having a transparent substrate approximately 0.1 mm in thickness, with a single-layer storage capacity of approximately 25 Gbyte. The laser beam used for recording or reproduction of a BD has a wavelength of approximately 405 nm, and the numerical aperture NA of the objective lens is 0.85. As shown in FIG. 10, an optical disk 600 according to the BD format specifications is configured having a recording layer 651 provided on a 1.1 mm-thick disk substrate 602, with a 0.1 mm-thick transparent cover layer 601 affixed so as to cover the recording layer 651. The total thickness of an optical disk according to the BD format specifications is 1.2 mm, or the same as a CD. Recording to or reproducing from the recording layer 651 is performed using an objective lens 671 having a numerical aperture NA of 0.85, where laser beam 681 is condensed onto the recording layer 651 from the transparent cover layer 601 side.
With BDs, an increase in coma aberration caused by using short-wavelength laser beam and a high-NA objective lens is suppressed by reducing the thickness of the transparent cover layer 601, through which the laser beam 681 passes, to approximately 0.1 mm.
Meanwhile, an HD DVD is an optical disk having a single-layer storage capacity of approximately 15 Gbyte, for which the numerical aperture NA of the objective lens is 0.65. Similar to a DVD, in an HD DVD, two 0.6 mm-thick substrates are layered together, and thus the total thickness of the optical disk is 1.2 mm, or the same as a CD.
There are three types in each of these optical disk formats: a read-only type using prepits as the recording layer; a write-once, read-many type using a pigment film as the recording layer and capable of being written to only once; and finally, a rewriteable type, which uses a phase-change film as the recording layer.
As described thus far, optical disks, at present, include CDs, DVDs, BDs, and HD DVDs, each having a different configuration. Optical disk configurations that ensure compatibility between optical disk types, such as, for example, between BD and HD DVD, have been disclosed in JP2006-196039A, JP2006-236509A, Published U.S. Patent Application No. 2006/0179448, and so on.
FIG. 11 schematically shows a cross-section of an optical disk 19000 compliant with both the BD and HD DVD format specifications.
The optical disk 19000 is configured so as to include a 0.1 mm-thick transparent cover layer 19010, on which is formed a BD recording layer 19510 compliant with the BD format specifications; a 0.5 mm-thick intermediate layer 19020 on the cover layer 19010; and on the intermediate layer an HD DVD recording layer 19520 which is, compliant with the HD DVD format specifications. Here, when recording to or reproducing from the BD recording layer 19510, 405 nm-wavelength laser beam 19810 is condensed onto the BD recording layer 19510 from the transparent cover layer 19010 side using an objective lens 19710 that has a numerical aperture NA of 0.85. On the other hand, when recording to or reproducing from the HD DVD recording layer 19520, 405 nm-wavelength laser beam 19820 is condensed onto the HD DVD recording layer 19520 from the transparent cover layer 19010 side using an objective lens 19720 that has a numerical aperture NA of 0.65.
Incidentally, with the demand for high-capacity, high-density optical disks increasing, the development of optical disks that can meet these demands is being pursued. For example, it is known that using near-field light, in which the propagation distance is shorter than the wavelength of the light, makes it possible to reduce the diameter of the beam spot to a fraction of the wavelength, thus allowing recording or reproduction at higher densities. The technology represented by FIG. 12 can be given as an example of recording using near-field light. Here, an objective lens 871, known as a solid immersion lens (SIL) and having a numerical aperture NA of approximately 1.5 to 2.5, is used. Laser light 881 is irradiated into the objective lens 871, and near-field light that emerges from the bottom surface of the objective lens 871 is condensed onto a recording layer 851 formed on the surface of an optical disk 800; whereby recording or reproduction is performed in this manner. Meanwhile, FIG. 13 shows a Super-RENS (Super-Resolution Near-Field Structure) system, in which a layer called a super-resolution film 961 is formed close to a recording layer 951 of the optical disk. The Super-RENS system is a system that records information to or reproduces information from the nearby recording layer 951 using near-field light obtained by irradiating laser beam 981 onto the super-resolution film 961 through an objective lens 971.
Conventional technology has proposed optical disks that are compliant with both the BD and HD DVD format specifications. However, optical disks and the like that are compliant with both conventional format specifications such as CDs, DVDs, Blu-ray Discs, and HD DVDs and format specifications of next-generation high-density optical disks such as the abovementioned near-field optical recording system are not yet known. Furthermore, conventional optical disk drives are not capable of recording/reproducing to/from next-generation high-density optical disks. Therefore, even if next-generation high-density optical disks are developed, users who have optical disk drives capable only of recording/reproduction to/from conventional optical disks will not be able to use such high-density optical disks.